fix: disable module output interpolation when not creating dlq (#62)

## Description
Disable the interpolation of the dlq alarm description when not creating
one as that leads to null string interpolation errors. Also use the
correct alarm description variable for dlq alarm descriptions.

cloudwatch.tf

@@ -2,7 +2,7 @@ locals {
   alarm_description     = var.alarm.description != null ? var.alarm.description : "SQS Queue Metrics: https://${module.this.aws_region}.console.aws.amazon.com/sqs/v2/home?region=${module.this.aws_region}#/queues/https%3A%2F%2Fsqs.${module.this.aws_region}.amazonaws.com%2F${module.this.aws_account_id}%2F${module.sqs.queue_name}"
   alarm_topic_arn       = var.alarm_topic_arn != null ? var.alarm_topic_arn : "arn:aws:sns:${module.this.aws_region}:${module.this.aws_account_id}:${module.this.environment}-alarms"
   dlq_alarm_enabled     = var.alarm_enabled && var.dlq_alarm_enabled
-  dlq_alarm_description = var.alarm.description != null ? var.alarm.description : "SQS Queue Metrics: https://${module.this.aws_region}.console.aws.amazon.com/sqs/v2/home?region=${module.this.aws_region}#/queues/https%3A%2F%2Fsqs.${module.this.aws_region}.amazonaws.com%2F${module.this.aws_account_id}%2F${module.sqs.dead_letter_queue_name}"
+  dlq_alarm_description = var.dlq_alarm.description != null ? var.dlq_alarm.description : (var.dlq_enabled ? "SQS Queue Metrics: https://${module.this.aws_region}.console.aws.amazon.com/sqs/v2/home?region=${module.this.aws_region}#/queues/https%3A%2F%2Fsqs.${module.this.aws_region}.amazonaws.com%2F${module.this.aws_account_id}%2F${module.sqs.dead_letter_queue_name}" : "")
 }
 
 resource "aws_cloudwatch_metric_alarm" "backlog" {